Don’t stress too much, I’m sure you managed to get at least a 6!
Don’t forget that all exams factor in, so even if you thought you did badly on writing you might have done amazing on listening or reading.
Yes you can resit the GCSEs but to do that you would have to take it a year later( so essentially resitting y11) so if you did really badly on that subject it might be worth reconsidering your a levels.